Traditional captioning devices, such as televisions and display monitors, combine text with a video broadcast, in which the text represents a transcribed audio portion of the video. The captions can be either closed captions or open captions. For closed captions, the caption data is embedded in a video signal, and the captions are not displayed unless they are decoded. The text or caption data is embedded in line 21 of the vertical blanking interval (VBI) in analog television. The VBI is an interval in a television signal that temporarily suspends transmission of the signal for an electron gun to move to the first line of a television screen for the next screen field. The VBI can be used to carry data, such as a test signal or the closed-captioning data.
Closed captioned data created for analog television may use protocols described by the EIA (Electronics Industry Association) 608 Standard. Closed captioned protocols for digital television may be described in the ELA 708 Standard. In a digital transmission, such as with high-definition television, the EIA 708 captioning data can be encoded in user data packets in an MPEG compressed video stream.
When captions are decoded, the captions can be displayed on the screen. Typically, captions can be decoded at the display device. Open captions may be already decoded in the video signal and may be displayed on screen, such as subtitles in foreign films. In broadcast transmission systems, caption data may be transmitted to an encoder that may include text and control codes.
Caption data may also be provided to the internet using transcribed data from broadcast sources, such as broadcast radio or television signals. For instance, a caption writer for a radio station may transcribe a broadcast program from the radio station to post on the radio station's web page. In another example, a caption writer may caption the audio portion of a television signal and send the caption data to an encoder device at a broadcast facility for inclusion in the video transmission.